Atmospheric air is nearly 80% nitrogen to start with. There have been plenty of studies into the use of 100% nitrogen for tyres on road going cars, and all suggest that any actual benefit is minimal at best.
My own experience of using atmospheric air is that there is little if any loss in pressure over time unless the tyre itself is damaged, or the rim is damaged or corroded around the bead area. I regularly check my pressures, and rarely have to add any air (other than a little every now and then to replaced what is let out in the process of testing the pressure.
I work in agriculture, where the kind of tyre pressures we use for some field applications may be as low as 6 psi in tyres which would otherwise be run at 16 to 24 psi for road work (at up to 25 mph) , and we don't suffer any significant (or measurable) pressure loss through the tyre compound (and at 5 psi, any loss would quickly become apparent and we would end up with a severely damaged tyre).
Added to which, any small loss in pressure (by whatever route) is largely irrelevant when put in the context of the (surprisingly wide) variation in the accuracy of the DIY type of pressure guages used by most motorists (if they bother about setting and maintaining the correct pressures to start with). The type of gauges integrated with the air line which most garages use/provide are well known for their inaccuracy; often giving varying pressure figures on repeat tests on the same tyre (without air being added or removed).
In motorsport, tyre pressures are more critical, and the pressure gauges used are much more precise (and much more expensive!).
Ordinary motorists paying for having their tyres nitrogen filles is a convenient way of parting the gullible from their money.